Various techniques for production of ceramics have been proposed previously. In particular, an injection molding process has attracted attention in recent years which can produce great quantities of molded articles of complex shapes. Specifically, the injection molding process comprises molding a ceramic-making material into a desired shape by an injection molding machine, heating the resulting molded article to decompose and volatilize (dewaxing) components of the material other than a ceramic powder or a metal powder, and sintering the molded article to form a ceramic. The ceramic-making material used at this time is obtained by mixing the ceramic powder and/or the metal powder with a binder and as required, a lubricant, etc. The binder is of particular importance and imparts strength to the molded articles obtained by injection molding. It is generally classified into water-soluble binders and synthetic resin binders.
The water-soluble binders, such as polyvinyl alcohol, polyvinyl butyral, polyethylene glycol, methyl cellulose, carboxymethyl cellulose, ethyl cellulose and hydroxypropyl cellulose, have a lower viscosity than the synthetic resin binders and penetrate well among the ceramic particles. But since they have poor lubricity, they are, in practice, diluted with water to impart lubricity. The strength of a molded article obtained by injection molding using the water-soluble binders is gradually exhibited as water is evaporated, and immediately after molding, the molded article has a very weak strength and is liable to undergo deformation. There may be a means of increasing the strength by decreasing the amount of the diluting water. However, since the flowability of the molding material is reduced, the injecting pressure of the injection molding machine increases or the ceramic powder causes wear and contamination of the molding machine. It is difficult actually to put such means into practice.
The synthetic resin binders, on the other hand, are better than the water-soluble binders in regard to imparting of strength, and are frequently used in injection molding.
Use of the synthetic resin binders in the production of ceramics is disclosed, for example, in U.S. Pat. Nos. 2,446,872, 2,593,943; 2,593,507; and 3,125,618.
U.S. Pat. No. 2,446,872 discloses a process for producing a ceramic product by using ethyl cellulose and shellac as binders.
U.S. Pat. Nos. 2,593,943 and 2,593,507 disclose processes for producing powdery products using polymonolefin thermoplastic resins such as polybutene.
U.S. Pat. No. 3,125,618 discloses a process for producing a powdery ceramic composition containing a resinous binder and consisting essentially of a homopolymer or copolymer of acrylic or methacrylic acid or its ester or amide.
However, all the processes disclosed in the specifications of the above U.S. Patents are disadvantageous in regard to decomposability and volatilizability at low temperatures and the appearance of the molded article after dewaxing. Thus, where a synthetic resin binder is used, the molded article is heated to a very high temperature to decompose the binder and at the same time the temperature slowly raised in order to promote dewaxing ability. Consequently, the time spent for the dewaxing step exceeds 100 hours, and sometime reaches as long as 300 hours.
None of the U.S. Patent specifications mentioned above disclose a ceramic-making composition containing a polyalkylene carbonate as the synthetic resin binder.